Document GB 1 505 873 describes a helmet-mounted optical display device for the presentation of a visual display to a helmet-wearing observer, where a reflective visor is mounted to the front of the helmet at such a distance that it lies in the observer's field of view, and where a conventionally generated hologram is formed on or carried by the visor. A reconstructing light source can be disposed in each side section of the helmet at such a distance to the reflective hologram that if the hologram is illuminated, the image thereon can only be reconstructed as a small representation in the observer's field of view.
One problem with this arrangement is that the visual reconstructions are limited to small representations; the helmet-mounted optical display device cannot be used for encoding and visualisation of large objects, let alone movie scenes.
Document U.S. Pat. No. 5,257,094 describes a helmet-mounted display device which comprises optical elements, e.g. mirrors and beam splitters, and holographic elements in its front section, which aim to improve permeability as regards both externally provided quantities and superimposed display information from two miniature cathode ray tubes disposed at the side sections. The display device further comprises a combined focussing lens. The optical link between the information-providing screen of the miniature cathode ray tubes and the corresponding observer pupils is designed such that the weight of the headgear including the display device is reduced.
One problem is that the observer must carry a rather heavy load on his head, because of the heavy overall weight of the headgear, despite the efforts which have been made in order to reduce the weight of the display device components mounted on the headgear. The display device is limited to the given arrangement of prefabricated optical and holographic components.
Document U.S. Pat. No. 4,933,755 describes a head-mounted stereoscopic display device, where a left and a right stereo image are represented on liquid-crystal displays for the right and left eye, respectively. This makes it possible to watch stereoscopic video images without the problem of an optical separation of those images for the respective eye, which typically occurs in direct-view displays.
Further, document U.S. Pat. No. 6,674,493 describes a head-mounted display device, where a common liquid crystal display is provided for both eyes, and where the stereo image is presented sequentially to both eyes through an active beam splitter.
One problem of stereoscopic head-mounted display devices is that although they are able to generate a stereo image, this stereo image exhibits all the drawbacks of stereoscopic image generation, in particular the missing possibility of an accommodation of the eye to an object with true depth, as are provided in holographic display devices, where three-dimensional objects are reconstructed based on computer-generated video holograms.
Further, document DE 10 2004 063 838 A1 describes a method and a device for computing computer-generated video holograms, where the reconstructions of the holograms encoded on a light modulator can be viewed from observer windows, and where there is a certain complex amplitude and phase distribution in those observer windows which is identical to the light which would be imaged by a real three-dimensional scene to the same position. The observer views the reconstruction of the three-dimensional scene through the observer window, where there are generally two separate observer windows, one for the left eye and one for the right eye.
The computation of the wave fronts in the observer windows is carried out by virtually slicing a three-dimensional scene within a frustum, i.e. a frustum-shaped space, into section planes, by computing the light propagation from those planes into an observer plane by way of transformations, and by summing them up there in an observer window. The size of the observer window can be confined to about the size of an eye or eye pupil.
During the reconstruction, the wave fronts which are necessary to make a certain three-dimensional representation - objects and/or scenes - visible in the observer windows can be generated in two different ways:                first, directly by encoding the complex wave front of the three-dimensional representation on a light modulator, which is situated outside the observer plane, and which is imaged into the respective observer window; or        secondly, indirectly by encoding the transformation of the complex wave front of the three-dimensional representation as a hologram on a light modulator, which is again situated outside the observer plane, and by back-transforming the complex wave front into the respective observer window.        
In either case, the information encoded on the light modulator is imaged or back-transformed into the observer window through an optical system, which is referred to as a screen. Usually, it is desired to realise a screen area which is as large as possible, e.g. 20 inches or more, and observer distances as with desktop displays or TV sets. Either a transform of the complex wave front encoded on the light modulator, as in the former case, or an enlarged image of the hologram encoded on the light modulator, as in the latter case, are generated on that area.
In either case, the frustum is defined by the viewing angle of the observer looking on to the screen surface. The observer shall be situated at such a distance to the screen surface as if he was using a desktop display or TV set. In both types of holographic direct-view displays, an observer can move relative to the screen surface, and he can watch a reconstruction of the three-dimensional representation from various positions.
Known holographic direct-view displays exhibit for example the following problems: they require the position of the observer window to be changed in space as the observer moves, i.e. it becomes necessary to track the observer window, and to provide all the devices necessary for this, such as components for detecting the eye position and optical components for tracking the observer window. Further, a large screen area is required. This makes the manufacture of holographic display devices relatively complicated and expensive.